(1) Field of the Invention
The present invention relates to a process for the preparation of a faujasite type zeolite, especially a faujasite type zeolite having a high silica content.
(2) Description of the Prior Art
A faujasite type zeolite ordinarily has an oxide molar composition represented by the following formula: EQU 0.9.+-.0.2M.sub.2 O.Al.sub.2 O.sub.3.xSiO.sub.2.wH.sub.2 O
wherein M stands for an alkali metal cation, x is a number of from 2.5 to 6, and w is a number of from 6 to 9. Faujasite type zeolites are used as a cracking catalyst for petroleum, and an adsorptive separating agent for use in separation of paraxylene or the like.
Ordinarily, these catalysts or adsorptive separating agents are required to have good heat resistance and acid resistance. In case of a faujasite type zeolite, the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio has close relations to these properties, and a faujasite type zeolite having a higher SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is excellent in these properties.
A faujasite type zeolite having a relatively low silica ratio, that is, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 2.5 to 4, is poor in the heat resistance and the acid resistance because of this low silica molar ratio, and therefore, faujasite type zeolites having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4 are widely used industrially and high-silica faujasite type zeolites having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4.5 are especially desired. However, it has been considered difficult to synthesize faujasite type zeolites having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4.5, especially at least 5, on an industrial scale with a good reproducibility. In the synthesis of faujasite type zeolites, the intended zeolites are produced only under very restricted conditions. Accordingly, the kinds and quantitative ratios of starting materials should be strictly and carefully selected and the reaction should be carefully carried out while strictly controlling such conditions as the mixing state, the degree of aging, the reaction temperature and the reaction time. These difficulties are increased when it is intended to prepare a faujasite zeolite having such a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio as described above and having a high purity.
Various processes have heretofore been proposed for preparing high-silica faujasite type zeolites. According to a typical process disclosed in Japanese Examined patent publication No. 36-1639 or No. 42-16941, a silica source such as silica sol, silica gel or finely divided solid silicic acid, sodium aluminate and sodium hydroxide are used as the starting materials and these starting materials are reacted under strictly controlled conditions to form a faujasite type zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4. In this process, however, it is indispensable that expensive amorphous solid silica should be used as the silica source and therefore, a faujasite type zeolite cannot be produced at a low cost.
Furthermore, a fatal defect is encountered when it is intended to synthesize a faujasite type zeolite in an industrial scale with a good reproducibility. Namely, it is impossible to carry out the reaction while stirring and mixing the reaction system. Especially when stirring is effected at the step of converting an amorphous reaction product of an aqueous gel, formed through long-time aging after mixing of the starting materials, (this product is considered to be a precursor before crystallization) to a faujasite type zeolite, crystallization to the intended faujasite type zeolite is inhibited and the majority of the product is converted to an aluminosilicate mineral of no practical utility such as a naturally occurring phillipsite mineral analogues (hereinafter referred to as "phillipsite") and gmelinite. Accordingly, in order to synthesize a high-silica faujasite type zeolite having a high purity, it is indispensable that the synthesis should be carried out while the reaction mixture in the form of an aqueous gel is allowed to stand still and mixing under stirring shoud be avoided. However, if this synthesis process under the stationary condition is carried out on an industrial scale, is required to effect the heat transfer for maintaining the reaction temperature, stirring is not conducted and, thus, the heat transfer in the reaction system becomes difficult and the temperature becomes uneven, resulting in such a disadvantage as simultaneous formation of phillipsite and gmelinite. Various means are adopted for avoiding simultaneous formation of phillipsite and gmelinite in the abovementioned stationary synthesis process, but the problem of simultaneous formation of phillipsite and gmelinite has not been completely solved.
Moreover, as means for reducing the manufacturing cost of a high-silica faujasite type zeolite, there have been proposed various improved processes. For example, Japanese Examined patent publication No. 49-13720 discloses a process for preparing a faujasite type zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4 by using as the silica source cheap sodium silicate which has been considered to be unable to give a high-silica faujasite type zeolite, wherein an aqueous mixture comprising hydrous solid silica, sodium aluminate and sodium hydroxide is aged to form an intermediate product and a small amount of this intermediate product is added as a crystal nucleus to a gel formed from a second aqueous mixture comprising sodium silicate and sodium aluminate.
Furthermore, Japanese Unexamined patent publication No. 47-4866 discloses a process for preparing a faujasite type zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 4 by using cheap sodium silicate, wherein an amorphous nucleus-forming core substance (seed) which has been prepared in advance and has an average particle size of 0.01 to 0.1 .mu.m is added to a reaction mixture comprising sodium silicate, sodium aluminate and aluminum sulfate.
Moreover, Japanese Examined patent publication No. 53-33556 discloses a process for preparing a faujasite type zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 4.5, wherein a mixture of sodium silicate and kaolin is used as the silica and alumina source and a zeolite seed prepared in advance from an aqueous mixture of sodium silicate and sodium aluminate is added to the above mixture.
In each of these processes, a specific combination of the starting materials is used and specific means such as addition of a seed is adopted. In these processes, the manufacturing cost can be reduced by using cheap sodium silicate as the silica source, but the process steps become complicated as compared with the steps adopted in the conventional processes and hence, these processes are not especially preferred from the industrial viewpoint. Moreover, the problem how to effect the synthesis under stirring of the reaction mixture for a large-scale production has not been solved in these processes.